Some of sensors, which have sensitivity in a visible light region having a wavelength of 400 nm to 700 nm, are referred to as optical sensors or visible light sensors. Optical sensors or visible light sensors are known to be used for, for example, detecting optical signals to read data, detecting the ambient brightness to control operation of electronic devices, and the like.
For example, in cellular phones or television devices, optical sensors are used for controlling the luminance of display screens in accordance with the ambient brightness of places where they are set.
Such semiconductor devices as optical sensors and visible light sensors are obtained by forming transistors over a glass substrate or a wafer and then cutting (dividing) the substrate.
A substrate is generally divided as follows: first, a groove (also referred to as a scribe line) is formed on the surface of the substrate by using a scribing device; then, the substrate is forcibly divided along the groove by using a cutting device. In the case of using a laser beam, first, a substrate is selectively irradiated with a laser beam to be locally heated. Next, the surface of the heated substrate is locally cooled by a cooling medium. Then, a crack is formed by utilizing thermal stress generated in the substrate, whereby the substrate is divided (for example, see Reference 1: Japanese Published Patent Application No. 2001-64029).